This is why.
These fertilizers promote growth. You get huge plants from these. Which is why so many people use them, I'm assuming. And the theory has been that it also helps build up carbon in the soil. We want that. It's all good, right?
|This Is big growth for the desert. :-)|
Turns out, that may be a load of bull crap...or rather, synthetic bull crap. Researchers led by professors Richard Mulvaney, Saeed Khan, and Tim Ellsworth, from the University of Illinois, are finding evidence that synthetic nitrogen does not work like we thought it did.
"The net effect of synthetic nitrogen use is to reduce soil’s organic matter content. Why? Because... nitrogen fertilizer stimulates soil microbes, which feast on organic matter. Over time, the impact of this enhanced microbial appetite outweighs the benefits of more crop residues."
"As organic matter dissipates, soil’s ability to store organic nitrogen declines. A large amount of nitrogen then leaches away, fouling ground water in the form of nitrates, and entering the atmosphere as nitrous oxide (N2O), a greenhouse gas with some 300 times the heat-trapping power of carbon dioxide. In turn, with its ability to store organic nitrogen compromised, only one thing can help heavily fertilized farmland keep cranking out monster yields: more additions of synthetic N."
"The loss of organic matter has other ill effects, the researchers say. Injured soil becomes prone to compaction, which makes it vulnerable to runoff and erosion and limits the growth of stabilizing plant roots. Worse yet, soil has a harder time holding water, making it ever more reliant on irrigation. As water becomes scarcer, this consequence of widespread synthetic N use will become more and more challenging." (http://grist.org/article/2010-02-23-new-research-synthetic-nitrogen-destroys-soil-carbon-undermines/ )
As you can imagine, this has been met with a lot of criticism. Companies that sell this stuff are not happy with this, as well as organizations which promote the use of this type of nitrogen as a good thing for crops. I'll leave it up to you to draw your own conclusions - check out the article above. It's good, and lists some of the critics as well, so you can go explore it yourself.
I have heard more criticism now that seems of more concern, so it really does seem to be something good to explore and make up your own mind over.